Pipelines carrying crude oil or other petroleum products typically require periodic cleaning since certain additives, contaminants and other materials within the transported fluids tend to settle and deposit within the pipeline. If these deposits are not removed, they may eventually restrict the flow through the pipeline, or may contaminate other products which are subsequently transported through the pipeline. It is, of course, impractical and uneconomical to dismantle a pipeline and clean the individual pipe sections. Instead, pipelines are typically cleaned by means of apparatus known as pipeline pigs.
In general, a pipeline pig is a device inserted into and forced through a pipeline, typically by means of fluid pressure. Pipeline pigs have been developed for a number of uses. For example, as mentioned above, pipeline pigs are employed to clean inside pipelines and remove sediment or debris. Pipeline pigs are also commonly used to segregate different fluids which may be simultaneously transported through a given pipeline.
Cleaning pigs are generally sized to fit tightly within the pipeline. Such pigs may include brushes disposed on the pig's outer surface which are designed to contact the inside surface of the pipeline and assist in dislodging undesirable sludge or deposits. Pipeline cleaning pigs generally have an outside diameter which is substantially the same as the inside diameter of the pipeline, although some pigs, such as those made of polyurethane foam, are compressible, such that their uncompressed diameter is greater than the inside diameter of the pipeline. In any such design, the tight fit of the pig facilitates proper cleaning of the pipeline.
When employed to clean a pipeline, the pigs are placed in service or "launched" by an apparatus generally known as a launcher. Similarly, a device known as a receiver or catcher is employed to recover the pig at the end of the pipeline section being cleaned. Pig receivers are generally permanently installed in the pipeline at some distance away from the launcher.
Present day pig receivers are not truly portable devices, but are permanently affixed at a particular location within a pipeline. Once installed, such receivers cannot readily be detached from the pipeline and used in another pipeline, or even repositioned for use at another location in the same pipeline.
Although entire pipelines may at times require cleaning, it is not uncommon for pipelines to develop restrictions or undesirable deposits only in particular lengths or segments. To save the time and expense of launching a pipeline pig and running it through the entire length of the pipeline, it is often desirable to clean only the restricted or deposit-ladened sections of the pipeline. Frequently, such restricted segments may be located a great distance away from the nearest installed receiver. In such instances, to expedite the cleaning process, it is common to disconnect and isolate the section of pipeline that contains the unwanted sediment or restriction from the rest of the pipeline and to launch the pig through the now-isolated pipeline section. These sections are frequently cleaned without employing a pig receiver on the now-opened end of the isolated pipeline section.
As stated above, cleaning pigs are typically forced through the pipeline by means of fluid that is injected behind the pig at high pressures. The force acting on the pig and propelling it through the pipeline may be many thousands of pounds. For example, with a 30" diameter pipeline and a fluid pressure of 100 psi, the force on the 30" diameter pig will exceed 70,000 pounds. With forces of this magnitude propelling the pig through the pipeline, where a pig catcher is not employed, the pig will be discharged from the end of the pipeline segment at an extreme velocity. Such uncontrolled high velocity ejection of the cleaning pig can pose a grave danger to personnel and equipment.
Accordingly, there remains a need in the art for an apparatus for safely capturing pipeline pigs when they are discharged from a pipeline at locations where a permanent receiver has not been installed. Ideally, such apparatus would be portable and reusable at different locations along the pipeline as pipeline sections are selectively cleaned. The apparatus should also be inexpensive and simple to attach and detach.